Approximately half of the people in the United States rely on groundwater as their primary source of drinking water. Unfortunately, groundwater contamination is very common. Major sources of groundwater contamination include natural sources, underground storage tanks, septic tanks, hazardous waste sites, landfills, chemical storage facilities, chemical spills, and agricultural activities. Man-made/anthropogenic sources are of particular concern as exposure to these types of contaminants can cause cancer or other serious health impairments in people.
The U.S. Environmental Protection Agency estimates that volatile organic chemicals (VOCs) are present in one-fifth of the nation's water supplies. VOCs are a class of chemical compounds that can often result in significant environmental impacts due to the ease the VOCs have migrating between soil and water. As such, once released at or near the ground surface, the VOCs often migrate into and through the unsaturated soils or rock of the vadose zone before impacting the underlying saturated intervals of the groundwater.
The vadose zone generally consists of porous to slightly porous materials that are often present in horizontal layers. Porosity levels vary from layer to layer with the conductance of liquids and gases usually much greater in the horizontal than vertical directions. Conductance of liquids and gases is also usually much greater in the horizontal than vertical directions in the saturated materials located below the water table in a zone commonly defined as a groundwater aquifer. After migrating into groundwater, volatile and semi-volatile materials can also evaporate back into the vadose zone and become part of the soil gas if favorable conditions exist. Favorable conditions are often dominated by permeable materials, where contaminants have concentrations greater than 1% of the contaminant's solubility levels, have a Henry's Law Constant greater than 1*10−5 atm-m3/mole and have vapor pressures greater than 0.1 mm Hg.
Under static subsurface conditions, soil gas and contaminants in and around the soil gas, will ultimately be homogeneous as the soil gas and contaminants reach toward a state of equilibrium. However, volatile and semi-volatile materials evaporating from groundwater will migrate into and through the vadose zone if a pathway conducive to the movement of the volatile and semi-volatile materials exists. If a porous pathway does not exist, the vapors will remain confined in semi-equilibrium with the soil and will slowly diffuse towards the ground surface.
Subsurface conditions can change, and soil gases in the soil will move toward a new state of equilibrium. Diffusion and convection are the two primary mechanisms responsible for the transport of soil gases migrating through the vadose zone. The moving forces for diffusion are the concentration gradients of the various compounds and elements, resulting in migration from zones of higher concentrations to zones with lower concentrations. Convective transport on the other hand occurs when the soil gases move through the soil pores under the influence of an external driving force. The convective forces may include variations in barometric pressures, wind gusts occurring above the soil surface, as well as density driven transport caused by changes in subsurface temperatures and moisture content. The relative importance of these forces are difficult to differentiate and appear to strongly vary both temporally and under small scale site-specific conditions.
Regardless of the forces inducing soil gas flow, the soil also plays an important function in the transmission of gases due to its permeability. Unsaturated soils with low permeability of less than 10−7 cm2 often result in slower diffusive processes dominating whereas with higher permeability a much faster convective transport occurs. Further, under conditions where the vertical permeability and conductivity of the unsaturated zone are mechanically increased, such as through agricultural tilling, faster conductive processes have historically been reported to occur, at least in the uppermost and near surface intervals (Renault, P., Mohrath, D., Gaudu, J. C., Fumanal, J. C., 1998, Air Pressure fluctuations in a prairie soil, Soil Science Society of America Journal, v. 62, pp. 553-563).
Prior known remediation systems and methods to treat contaminated soil and groundwater include two main types: in-situ treatments that primarily treat contamination below the ground surface, and treatment processes that occur mostly above the ground surface. All of these prior known soil and groundwater remediation systems and methods require expensive engineered treatment wells and above ground infrastructure to support or control the remediation activities or the drilling of boreholes where expensive chemicals are injected or energy sources are used to treat the subsurface contamination. The engineered treatment wells include cased wells that extend into the vadose zone and may extend into the contaminated groundwater. The cased wells have casings that have smaller diameters than the boreholes and often extend several feet above ground.
The time needed to complete the soil and/or groundwater remediation often takes years to decades to complete, due to incorrect remedial designs, inefficiencies of the various treatment systems, and/or equipment failures. Modifications to improve the efficiency of these treatment systems, after the systems are installed, are often difficult to identify and/or extremely expensive to implement. Further, during the remediation period the impacted properties rarely can be used for any other purpose other than the operation of the treatment system.
The cased well casings in the prior known groundwater remediation systems and methods have a screen section that extends upwardly a selected distance from the bottom end of the borehole. These cased wells generally have engineered porous fill material located between the screen and the borehole wall from the bottom end up to slightly above the top of the screen section. Above the fill material substantially impermeable materials seal the space between the casing and the borehole wall. Fluids can only travel up or down the inside of the casing from the screened section to treatment systems located above the ground surface and usually located at some distance from the treatment wells.
The wells associated with prior known remediation systems are: 1) constructed to treat only the single zone adjacent to the screened well section; 2) not capable of allowing the area-wide mixing of atmospheric gases and temperatures into the vadose zone to enhance the remedial influence of barometric pumping; 3) not used as a single remedial treatment method to remove volatile and semi-volatile contaminants from impacted soil and groundwater; 4) not able to depress the volatilization potential of soil gas contaminants during colder weather; and 5) limited in location as the above ground components are restricted to areas where near ground obstructions are not a hazard to transportation systems, pedestrian or recreational uses.
The prior known remediation systems include various blowers, pumps and other components that are often expensive. Power to run the mechanical and electrical components, as well as the ongoing equipment maintenance activities adds to the cost. In addition, regulatory requirements in many states require that treatment wells and other components be removed after the systems are no longer in use, adding to the costs associated with operating these prior known remediation systems.